Background

Nematode infections in humans include ascariasis, trichuriasis, hookworm, enterobiasis, strongyloidiasis, filariasis, trichinosis, and angiostrongyliasis (rat lungworm disease), among others. The phylum Nematoda, also known as the roundworms, is the second largest phylum in the animal kingdom, encompassing up to 500,000 species. Members of Nematoda are elongated, with bilaterally symmetric bodies that contain an intestinal system and a large body cavity.

Many roundworm species are free living in nature. Recent data have demonstrated that approximately 60 species of roundworms parasitize humans. Intestinal roundworm infections constitute the largest group of helminthic diseases in humans. According to a 2005 report by the World Health Organization (WHO), approximately 0.807-1.221 billion humans have ascariasis, 604-795 million have trichuriasis, and 576-740 million have hookworm infections worldwide.
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Pathophysiology

The life cycle of parasitic nematodes is clinically important. Some nematode infections can be transmitted directly from infected to uninfected people; in others, the nematode eggs must undergo a process of maturation outside the host. In a third category, the parasites may spend a part of their life cycle in the soil before becoming infective to humans.

As with other parasitic infections, definitive diagnosis of nematode infections depends on demonstration of the stage of the life cycle in the host. Nematodes, as with most other worms infectious to humans, almost never complete their entire life cycle in the human host.

The life cycles of nematodes are complex and highly varied. Some species, including Enterobius vermicularis, can be transmitted directly from person to person, while others, such as Ascaris lumbricoides, Necator americanus, and Ancylostoma duodenale, require a soil phase for development. Because most helminthic parasites do not self-replicate, the acquisition of a heavy burden of adult worms requires repeated exposure to the parasite in its infectious stage, whether larva or egg. Hence, clinical disease, as opposed to asymptomatic infection, generally develops only with prolonged residence in an endemic region.

Unlike with protozoan infections, a casual or a low degree of exposure to infective stages of parasitic nematodes usually does not result in patent infection or pathologic findings. Repeated or intense exposure to a multitude of infective stage larvae is required for infection to be established and disease to arise. Anisakis species cause erosive and/or hemorrhagic lesions in or near the main lesion, forming a tunnel through the gastric mucosa to the submucosa.

Eosinophilia and elevated serum immunoglobulin E (IgE) levels are features of many nematode infections; when unexplained, these symptoms should always prompt a search for occult roundworm infection. Humans do not appear to develop significant protective immunity to intestinal nematodes, although the mechanisms of parasite immune evasion and host immune responses to these infections have not been elucidated in detail.

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Epidemiology

Frequency

United States

Trichuriasis: Recent estimates indicate that 2.2 million people in the United States are infected with Trichuris trichiura (whipworm), mainly in the rural Southeast.

Enterobiasis: This is the most common of all helminthic infections, with an estimated 42 million cases.

Ascariasis: An estimated 4 million people, mainly in the Southeast, are infected with Ascaris species.

Hookworm: This infection still has a low degree of prevalence in the Southeast.

Strongyloidiasis: A prevalence rate of 0.4-4% has been estimated in southern states.

Trichinosis: The prevalence rate is 4-20%.

Anisakiasis: Annually, fewer than 10 cases occur in the United States.

Angiostrongyliasis (rat lungworm disease): This is a rare infection that has been reported in Hawaii and Louisiana.
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International

Trichuriasis: Infection with T trichiura is one of the most prevalent nematode infections worldwide; approximately 800 million persons have trichuriasis worldwide, most abundantly in warm moist regions. Infection rates of up to 75% were found in young schoolchildren in Puerto Rico.

Enterobiasis: Pinworm is also highly prevalent throughout the world, particularly in countries of the temperate zone. Children are most commonly infected. Estimated prevalence rates among children in various world regions are 4–28%.

Ascariasis: Ascaris, or roundworm, infection is the common helminthic infection in humans, with an estimated worldwide prevalence of 1 billion. The causative organism, A lumbricoides, is cosmopolitan in distribution, being most abundant in tropical countries.

Hookworm: Human infection with the 2 species of hookworm, A duodenale and N americanus, is estimated to affect approximately 550-750 million people.
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Strongyloidiasis: The infection is more common in tropical countries with poor sanitation, especially in countries of Southeast Asia and parts of Africa. Strongyloides stercoralis is also endemic in Jamaica and presumably elsewhere in the Caribbean. An estimated 30-100 million persons worldwide have strongyloidiasis.

Trichinosis: Trichinella species are distributed throughout the world and are spread widely in nature among a large number of carnivorous animals, with humans acting as an incidental host. Trichinosis has been a major public health problem and has been reported in many Asian countries, including China, Japan, Korea, and Thailand.

Dracunculiasis: Estimates of the number of people infected with Dracunculus medinensis in Africa, the Middle East, India, and other tropical areas range from 50-150 million. An aggressive eradication campaign has been underway to eliminate D medinensis, which is called the Guinea worm.

Filariasis: An estimated 120 million people are infected with Wuchereria bancrofti, Brugia malayi, and Brugia timori.

Loiasis: Loa loa is irregularly distributed in Africa. It is estimated that between 3 and 13 million people in West and Central Africa are infected.

Onchocerciasis: Onchocerca volvulus infects 20 million people in West, Central, and East Africa and another 1 million people in scattered foci in Central America and South America. The disease caused by this filarial worm is called river blindness.

Anisakiasis: Approximately 20,000 cases of anisakiasis are reported annually worldwide; over 90% are from Japan and most others from Spain, the Netherlands, and Germany, depending on the habits of fish consumption.
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Angiostrongyliasis (rat lungworm disease): This is an uncommon infection that mostly affects the Pacific Basin and Southeast Asia. Isolated cases have been reported in the Caribbean, Africa, and Australia.
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Mortality/Morbidity

Nematode infections are usually asymptomatic or subclinical.

Strongyloidosis may be fatal in immunocompromised patients and in newborns.